470 Studies in the Theory of Descent. 



orders Diptera and Hymenoptera, as will be shown 

 subsequently, in sub-orders and tribes, it depends 

 upon the importance of the part of the body affected 

 by the predominant change. In the latter case 

 the number of changes is of no importance, 

 because these are so numerous that the difference 

 vanishes from our perception ; but an equal 

 number of changes, even when very great, may 

 now produce a much greater or a much smaller 

 transformation in the entire bodily structure 

 according as they affect typical or untypical por- 

 tions, or according as they keep in the same 

 direction throughout a long period of time, or 

 change their direction frequently. 



Those unequal form-divergences which occur in 

 the higher systematic groups a re always associated 

 with a different formation of groups the larvae 

 form different systematic groups to the imagines, 

 so that one of these stages constitutes a higher or 

 a lower group ; or else the groups are of equal 

 importance in the two stages, but are of unequal 

 magnitude they do not coincide, but the one 

 overlaps the other. 



Incongruencesof this last kind appear in certain 

 cases within families (Nymp/ialidte), but I will not 

 now subject these to closer analysis, because their 

 causes will appear more clearly when subsequently 

 considering the orders Hymenoptera and Diptera. 

 Incongruences of the first kind, however, admit 

 of a clear explanation in the case of butterflies. 



